In the atmospheric‑pressure plasma machining system for silicon carbide (SiC), process gases serve as the core medium for plasma generation, active particle excitation, and chemical reaction progression. Argon (Ar), carbon tetrafluoride (CF₄), and oxygen (O₂) perform independent yet coupled functions. Their flow ratios directly govern plasma jet stability, reactive fluorine particle concentration, etching reaction rate, and wafer surface forming quality, which is critical for resolving major mass‑production bottlenecks, including low etching efficiency, severe surface deposition, and poor machining uniformity.
Based on single‑factor controlled experiments conducted under standard conditions (room temperature, atmospheric pressure, and 350 μm thick SiC wafers), this paper systematically investigates the reaction mechanisms, flow‑dependent influence rules, and typical process pitfalls of the three process gases, providing standardized technical guidance for parameter debugging in SiC substrate plasma modification.

1. Argon (Ar): Stabilizing Carrier for Plasma — Higher Flow Does Not Yield Better Performance

As an inert excitation gas, Ar does not directly participate in SiC etching reactions, yet it acts as an essential carrier for stabilizing plasma jets and facilitating CF₄ pyrolysis. Compared with other inert gases, argon features lower cost, easier ionization, and superior discharge stability, making it the preferred carrier gas for plasma machining. Its flow rate determines the overall gas field distribution, electron density, and diffusion efficiency of active particles, thereby dominating the entire machining stability.
In this study, Ar flow gradients were set at 15, 17, 19, 21, 23, and 27 SLM. Other parameters were fixed: a machining power of 500 W, a machining distance of 4 mm, and a CF₄ flow rate of 60 SCCM. Fixed‑point dwell comparative experiments were carried out.

Figure 1: Effects of varying Ar flow rates on plasma machining performance
 
Experimental results indicate that the Ar flow rate has a negligible influence on the full width at half maximum (FWHM), which means it barely changes the effective machining range of plasma. However, both the peak material removal rate and volumetric removal rate decrease continuously with the increase in Ar flow.
Mechanistic analysis reveals that under low Ar flow conditions, the plasma jet presents a dark green appearance with a high density of internal reactive fluorine radicals, enabling sufficient etching reactions and optimal material removal efficiency. As the Ar flow increases continuously, excessive inert gas dilutes the concentration of active particles inside the plasma and consumes partial ionization energy, reducing CF₄ cracking efficiency and F* radical output. Furthermore, excess Ar disturbs the jet flow field and induces minor airflow turbulence, which further suppresses etching reactions and ultimately causes a significant decline in machining efficiency.
Process Conclusion: Excessively high Ar flow is unnecessary. Under the premise of stable and sustained plasma discharge, low Ar flow parameters are preferred to effectively improve the etching efficiency of SiC substrates.

2. Carbon Tetrafluoride (CF₄): Core Fluorine Source for Etching with an Optimal Flow Threshold

CF₄ is the exclusive source of reactive fluorine particles in the SiC plasma etching system. No effective SiC etching can be achieved without CF₄ supply, rendering it the core reactive gas of the machining process. Due to the high molecular bond energy of CF₄, its ionization and pyrolysis rely on a stable Ar plasma environment. Its flow rate directly determines the total generation of F* radicals and acts as the dominant variable affecting etching efficiency.
With fixed parameters (Ar flow: 19 SLM, power: 500 W, machining distance: 4 mm), CF₄ flow gradients ranging from 0 to 85 SCCM with a 10 SCCM interval were set to explore its influence on machining performance.

Figure 2: Effects of varying CF₄ flow rates on plasma machining performance
 
The experimental results show distinct staged variation characteristics:
(1) Low‑flow regime (0–65 SCCM): Increasing CF₄ flow continuously raises the concentration of F* radicals in the plasma, significantly promoting chemical reactions with silicon components on the SiC surface. Both peak removal rate and volumetric removal rate increase synchronously, while the FWHM remains stable without deterioration of machining uniformity.
(2) High‑flow regime (>65 SCCM): Excessive CF₄ breaks the original plasma discharge equilibrium, disturbs the ionization environment of Ar plasma, narrows the plasma discharge range, and restrains further CF₄ pyrolysis. In this regime, the growth of F* radical concentration slows down or even reaches saturation. Meanwhile, flow field stability declines, resulting in stagnant growth of peak removal rate and a prominent drop in volumetric removal rate.
Process Conclusion: Under the standard working condition of 500 W power and 19 SLM Ar flow, the optimal CF₄ flow threshold is 65 SCCM. At this parameter, the volumetric material removal rate of SiC reaches its maximum value, achieving a balance between high machining efficiency and excellent process stability.

3. Oxygen (O₂): Dual‑Functional Auxiliary Gas — Precision Flow Control Eliminates Surface Deposition Defects

Oxygen serves as an auxiliary process gas and does not directly participate in the core chemical reactions of SiC etching. Nevertheless, it plays a vital role in eliminating surface deposition, optimizing surface quality, and regulating etching efficiency. It is a key parameter for resolving common post‑processing defects of SiC substrates, including white spots, annular deposition, and out‑of‑tolerance surface roughness, with distinct dual‑sided effects on the machining process.
With fixed Ar flow (19 SLM), CF₄ flow (60 SCCM), power (500 W), and machining distance (4 mm), O₂ flow gradients ranging from 0 to 60 SCCM were set to systematically verify its process characteristics.

Figure 3: Effects of varying O₂ flow rates on plasma machining performance
 
(1) Oxygen inhibition regime (0–20 SCCM): Trace oxygen preferentially consumes reactive F* radicals in the plasma and suppresses etching reactions, leading to lower peak and volumetric removal rates compared with oxygen‑free conditions, which is an unfavorable process window.
(2) Optimal efficiency enhancement regime (20–40 SCCM): Appropriate oxygen reacts with fluorocarbon polymer fragments (CF, CF₂, CF₃) generated by CF₄ pyrolysis, fundamentally inhibiting the deposition of macromolecular polymers on the wafer surface and greatly reducing white annular deposition defects. Meanwhile, the reaction produces reactive OF* groups, which generate supplementary F* radicals through secondary pyrolysis to further improve etching efficiency. Specifically, the volumetric removal rate at 40 SCCM O₂ exceeds twice that under oxygen‑free conditions.
(3) Process instability regime (>40 SCCM): Excessive oxygen severely disrupts plasma discharge balance, causing jet flickering, jittering, and even flameout. The overall plasma stability decreases sharply, resulting in saturated etching efficiency and degraded machining uniformity.



Figure 4: Contour deviation of machined spots under different O₂ flow rates



Figure 5: Comparison of surface deposition on machining spots under different O₂ flow rates


Figure 6: Morphology of plasma jets under different O₂ flow rates
 
Microscopic morphology characterization shows that under oxygen‑free conditions, a large‑area white deposition ring with a diameter of 7.5 mm forms around the machining spot, severely degrading the wafer surface quality. After introducing 20 SCCM O₂, the diameter of the deposition ring is reduced to 6.8 mm with significantly decreased sediment density, enabling high‑quality surface formation without complicated post‑polishing procedures.

4. Core Process Summary of Gas Ratio Optimization

(1) Carrier gas (Ar): Prioritize plasma stability. Adopt the minimum feasible flow rate under uniform and stable discharge conditions to avoid active particle dilution.
(2) Fluorine source gas (CF₄): Strictly follow the optimal flow threshold to prevent ionization inhibition and efficiency attenuation caused by excessive gas supply.
(3) Auxiliary gas (O₂): Avoid low‑flow inhibition and high‑flow instability. The optimal process window is 20–40 SCCM, which synchronously achieves high etching efficiency and superior surface quality.
Gas ratio optimization constitutes the foundation of the plasma machining process. Machining power and torch‑to‑target distance can further amplify or weaken the effects of gas parameters. The optimal multi‑parameter coupling process combination will be comprehensively discussed in the subsequent orthogonal experiment optimization study.
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